When a molecular ion generated from a sample molecule is made to move through a gaseous (or liquid) medium by the effect of an electric field, the ion moves at a speed proportional to its mobility which is determined by the strength of the electric field, size of the molecule and other factors. Ion mobility spectrophotometry (IMS) is a measurement technique which utilizes this mobility for an analysis of sample molecules. Commonly known devices include a device in which ions separated according to their mobilities are introduced into and detected by a detector, as well as a device in which ions separated according to their mobilities are introduced into a mass analyzer or similar device to detect each ion after separating them according to their mass-to-charge ratios.
As one technique for the ion mobility spectrometry, a technique called the “differential mobility spectrometry” (DMS) has conventionally been known (see Patent Literature 1 and Non Patent Literature 1). Under a strong electric field, the mobility of an ion is no longer proportional to the strength of the electric field, and furthermore, the rate of change in the mobility is different for each ion species. DMS uses this principle to separate different ion species.
FIGS. 10 and 11 are schematic configuration diagrams of an ion separator section 1 in a conventional parallel plate electrode differential mobility spectrometer (DMS). FIGS. 10 and 11 illustrate the ion separator section 1 viewed from different directions.
In this ion separator section 1, the space sandwiched between upper and lower plate electrodes 11 and 12, both being parallel to the X-Z plane, serves as an ion separation space 15 for separating ions. From a pulse voltage generator 21, an asymmetric pulse voltage in which, as shown in FIG. 12, the period of time TH where the voltage value is at the high level (V1: the reverse polarity to the ions) is significantly different the period of time TL where the voltage value is at the low level (V2: the same polarity as the ions), is applied between the plate electrodes 11 and 12, which are held by spacers 13 and 14 at a predetermined distance from each other in the Y-axis direction. A flow of appropriate buffer gas, such as air, is formed at a constant flow velocity in the Z-axis direction within the ion separation space 15.
During the period of time TH where the applied voltage is at the high level, a high electric field having a relatively high field strength EH is formed within the ion separation space 15. During the period of time TL where the applied voltage is at the low level, a low electric field having a relatively low field strength EL is formed within the ion separation space 15. Now, consider the case where three kinds of ions with different mobilities are injected into the ion separation space 15 at a predetermined speed in the Z-axis direction, as shown in FIG. 10.
While the ions are moving forward, the strength of the electric field within the ion separation space 15 alternates between EH and EL. Consequently, the ions travelling through the ion separation space 15 alternately experience a force due to the high electric field with field strength EH and a force due to the low electric field with field strength EL. For the ions travelling through the ion separation space 15, this situation can be regarded as the presence of a high electric field region 100 with field strength EH and a low electric field region 101 with field strength EL alternately arranged along the Z axis, as shown in FIG. 10. The ratio of the length of the high electric field region 100 to that of the low electric field region 101 in the Z-axis direction is determined by the ratio of the high-level-voltage period TH to the low-level-voltage period TL.
When the ions pass through the high electric field region 100, the three kinds of ions all move closer to the upper plate electrode 11 due to the force from this electric field. In this phase, the three ions, whose mobilities within the high electric field region 100 are μH1, μH2 and μH3, have speeds of EH·μH1, EH·μH2 and EH·μH3, respectively. By comparison, when the ions pass through the low electric field region 101, the three kinds of ions all move closer to the lower plate electrode 12 due to the force from this electric field. In this phase, the three ions, whose mobilities within the low electric field region 101 are μL1, μL2 and μL3, have speeds of EL·μL1, EL·μL2 and EL·μL3, respectively. Consequently, each of the three ions moves in zigzags within the ion separation space 15.
For one kind of ion for which the ratio of the mobility within the high electric field region 100 to the mobility within the low electric field region 101 is α1=μH1/μL1 (which is hereinafter simply called the “mobility ratio”), if the separation condition is determined so as to satisfy TH·EH·μH1=TL·EL·μL1, the amount of movement of this ion species in the Y-axis direction within the high electric field region 100 equals that of the movement in the Y-axis direction (in the negative direction) within the low electric field region 101. Therefore, this ion species moves in zigzags along the central axis C while being carried by the buffer-gas flow, to eventually reach the exit end of the ion separation space 15. The path of this ion species is schematically indicated by line “a” in FIG. 10.
For the other two ion species whose mobility ratios are different from α1, i.e. α2=μH2/μL2 and α3=μH3/μL3, the amount of movement in the Y-axis direction within the high electric field region 100 is not equal to that of the movement in the Y-axis direction within the low electric field region 101. Therefore, as they move forward within the ion separation space 15, those ions gradually deviate from the central axis C (see trajectories “b” and “c” in FIG. 10). Upon reaching a sufficiently large distance from the central axis C, the ions come in contact with the plate electrode 11 or 12 and disappear. Thus, the DMS can isolate and extract only an ion species having a specific mobility ratio.
In the previously described parallel-plate electrode DMS, the ions passing through the ion separation space 15 experience the force due to the electric field only in the Y-axis direction (positive or negative). Accordingly, if an ion having the mobility ratio of α1 is incident on a point displaced from the central axis C on the entrance end plane (which is an end plane parallel to the X-Y plane) of the ion separation space 15, this ion will exit from the ion separation space 15 at a point displaced from the central axis C on the exit end plane of the same space. This means that, if a group of ions having the same mobility ratio are spatially spread when they are introduced into the ion separation space 15, those ions will remain spatially spread even when they exit from the ion separation space 15. Furthermore, even a group of ions having the same mobility ratio and incident on the same point on the entrance end plane of the ion separation space 15 do not always follow the path “a” along the central axis C, since they vary in the amount of initial energy, incident angle and other relevant quantities. Therefore, ions having one mobility ratio are typically spread over a wide area, e.g. as indicated by “A” in FIG. 11, when they arrive at the exit end plane of the ion separation space 15.
Such a spatial spread of the ions having the same mobility ratio lowers the transmission efficiency of the ions when the ions arriving at a specific position are to be extracted through an aperture plate, skimmer or similar device placed outside the exit end of the ion separation space 15. Consequently, it will be difficult to improve the measurement sensitivity. Furthermore, when a plurality of ion species having different mobility ratios are to be separated from each other and individually extracted, it is likely that other ion species will be mixed with the intended one and consequently lower the measurement accuracy.
As an improved version of the DMS, a device using cylindrical electrodes as shown in FIG. 13 has also been commonly known (see Non Patent Literature 2). In this DMS, as in the aforementioned example, an asymmetric pulse voltage is applied between the concentrically arranged cylindrical electrodes 110 and 120, creating an electric field similar to the previously described one within the ion separation space 15 between those cylindrical electrodes 110 and 120. However, in this device, the equipotential surfaces due to the electric field created between the pair of cylindrical electrodes 110 and 120 have a smaller radius of curvature at a closer position to the inner side, or to the cylindrical electrode 120. Therefore, a force which reduces the spatial spread of the ions travelling through the ion separation space 15 acts on those ions, making them converge on an arc-shaped specific center line B.
However, even such a DMS cannot produce a significant ion-converging effect in the lateral direction of the ion stream, and therefore, is not sufficiently effective for improving the ion transmission efficiency. Furthermore, producing the cylindrical electrodes with a high level of mechanical precision incurs a considerably high amount of cost as compared to the parallel-plate electrodes. Accordingly, a configuration as shown in FIG. 13 is unfavorable in terms of the device cost.